1. Field of the Invention
The present invention relates to probe cards and more particularly, to a vertical-type probe card for high frequency application.
2. Description of the Related Art
Following fast development of semiconductor technology, it realizes ICs (Integrated Circuits) micro-sized, versatile functions, and relatively faster processing speed and higher operation frequency. Therefore, the number of I/O (Input/Output) contacts on a multifunctional IC must be relatively high as well as on a semiconductor wafer. In consequence, a probe card capable of probing high-density contact array in wafer level test, such as a vertical-type probe card, becomes more and more important. Moreover, since electronic devices inside the ICs also tend to be operated in high speed and high frequency, electrical specifications of device characteristics under any possible bias condition, operation frequency, signaling mode, and etc. are critical. Therefore, designing a corresponding test probe card must consider all the critical factors and meet the transmission integrity of test conditions.
FIG. 1 shows a vertical-type probe card 1 according to the prior art. According to this design, a circuit board 10 of the probe card 1 passes high-frequency test signals 20 from a test machine (not shown) through a purity of coaxial transmission lines 11 to a probe set 12 to probe ICs 300 at a wafer 30. The coaxial transmission line 11 has an axial wire wrapped by a layer of dielectric material, which is in turn wrapped by a shielding metal electrically connected to the grounding potential of the circuit board 10 to maintain the characteristic impedance of the transmitted high-frequency test signal. The probe set 12 comprises an upper guide plate 121, a lower guide plate 122, and a plurality of vertical probes 120 inserted through the upper guide plate 121 and the lower guide plate 122 to keep standing on the circuit board 10 and to be respectively connected to the coaxial transmission lines 11 or other signal lines. According to this design, the probe set 12 is provided between the end of the coaxial transmission line 11 and the wafer 20, the upper guide plate 121 and the lower guide plate 122 are made of electrically insulating materials to prevent leakage current occurred among the probes 120, and each of the probes 120 is an individually conducting member without having an outer shielding metal like the coaxial transmission line 11. Therefore, when a high frequency signal is transmitted through the probe 120, a parasitic capacitor subject to the dielectric environment around the probe 120 will induce the dielectric loss of the high frequency signal. FIG. 2 is the frequency-characteristic curve of the high frequency signaling of the probe card 1. As illustrated, the return loss curve S22 shows the characteristic impedance mismatched of the high frequency signaling; the insertion loss curve S21 shows the passband frequency at −3 dB lowered than 200 MHz, which is far away from GHz segment of high frequency test specification.
FIG. 3 shows a probe structure 14 constructed according to European patent publication No. 0180013. According to this design, the probe structure 14 comprises a probe body 100 formed of a succession of body layers, and probes 102, 104 and 106 respectively inserted through the probe body 100. This design allows arrangement of an electrically grounded metal around the signal probe 102 to achieve the maintenance of characteristic impedance for the transmission of high frequency signal. However, because the probes 102, 104 and 106 are fitted into respective vias provided through the body layers and not flexible within the probe body 100, the hard metal structure of the probes 102, 104 and 106 may damage the test contacts on the test wafer or, the worst, destruct the internal circuit devices right below the test contacts when the probe structure 14 is operated to probe the test wafer. Therefore, this design of probe structure is not suitable for testing semiconductor wafers.
Therefore, it is desirable to provide a vertical-type probe card that maintains the characteristic impedance for transmission of high frequency signals without damaging to the ICs of the test sample.